11Alive Meteorologist Chesley McNeil said the clouds are very rare in the southeast, but more common in the midwest. He also says there's debate over what causes the phenomenon. Scientists agree it's a meeting of two fronts, but are split on whether it's caused by a cold/warm front or a dry/moist front. The Royal Meteorological Society is documenting their appearance to see if they deserve a separate cloud classification. If that happens, it would be the first cloud named since 1951.

"They're very ominous looking," McNeil said. "They're rolling, and that's where the name 'river of the sky' comes from." The name is roughly translated "roughened or agitated waves".